The invention relates to a bolometer for measuring the intensity or the presence of incident radiation.
A field of application of the invention is radiation sensors, for example used in infrared imaging or in infrared thermography at room temperature.
A bolometer conventionally comprises a variable temperature-dependent resistance and a body absorbing the incident radiation or another external signal. By measuring the variation of this resistance, it is possible to measure this temperature, with which itself the variation of the incident external signal may be traced back.
This type of sensor may be applied to radiometry, the radiations then absorbed at the surface of the bolometer heat up the bolometer and in this case, the measurement of the temperature of the bolometer gives an indirect measurement of the received power.
The operating principle of bolometers is based on thermal phenomena which are essentially relatively slow, even if miniaturization is contemplated with which the time constants may be reduced.
The thermal properties of the system are related to the heat conductance between the measurement element and a thermostat, as well as to the heat capacity of the element subject to the radiation. These parameters define the response time of the system.
Improving the performances of bolometers is a key point for increasing the sensitivity and response times of the targeted applications. These improvements in performances may be obtained by closing the loop of the system by means of heat feedback.
The document “A room temperature Si3N4/SiO2 membrane-type electrical substitution radiometer using thin film platinum thermometers”, G. Allègre, B. Guillet, D. Robbes, L. Méchin, S. Lebargy, S. Nicoletti, Institute of Physics Publishing, Measurement Science and Technology, 17(2006) 1-7, describes the feedback control principle by electric substitution of heat. In the absence of incident light, the temperature of the membrane forming the absorbing receiver is maintained constant at a value Tref by using a resistive heating device and a temperature control device. In the presence of light, the power required for maintaining the membrane at temperature Tref is provided by the resistive heating device and the chopped incident light. The incident power on the absorbing receiver may be estimated by measuring the equivalent electric power required for obtaining the same rise in temperature by using either one of the two heating methods.
This known device with heat feedback has the drawback of a large occupied volume, which acts to the detriment of the integration and miniaturization of the system. This drawback is critical in the case of high integration applications such as for example matrices of pixels which may be used for forming night vision cameras. Another drawback is the delay between the deposition of heat and the temperature measurement, which limits the performances of the system in a closed loop (gain margin and phase margin).
The invention is directed to obtaining a bolometer overcoming the drawbacks of the state of the art, and to improving the performances of the feedback control.
For this purpose, a first object of the invention is a bolometer, including an outer surface for thermal absorption of incident radiation, the absorption surface being in thermal contact with at least one element for measuring the incident radiation, having an electric measurement resistor, which is variable with temperature, the measurement element being located in a heat feedback loop including a corrector for applying heating power to a heating resistive means in order to maintain the temperature of the electric measurement resistance equal to a setpoint temperature, characterized in that the resistive heating means comprises the measurement element, the corrector is provided for generating a frequency component of the heating power, which is applied to a first coupling means provided between the measurement element and the corrector in order to apply to the measurement element a DC-free signal, a second coupling means distinct from the first coupling means is provided between the measurement element and a DC biasing means for maintaining the electric measurement resistance at a prescribed DC operation point.
According to other features of the invention,
The resistive heating means is formed by the measurement element.
The first coupling means is capacitive.
The heat feedback loop and the DC biasing means are provided for modifying the power dissipated in the measurement resistor without changing its operation point.
The first coupling means applies to the measurement element a signal in a frequency band above 20 kHz.
The heat feedback loop includes a means for forming an error signal between the signal present in the measurement element and a corresponding signal with a prescribed constant setpoint, the corrector being provided for applying to the measurement element via the first coupling means an alternating signal depending on the error signal.
The corrector is provided for applying to the measurement element via the first coupling means an alternating signal which is amplitude-modulated by the error signal.
The corrector is provided for applying to the measurement element via the first coupling means, a first sinusoidal signal which is amplitude-modulated by the error signal.
The corrector includes a means for linearizing the frequency component of the heating power according to the error signal.
The linearization means of the corrector includes a means for pulse-width modulation of the error signal in order to form an intermediate signal and a means for modulating the intermediate signal with a second sinusoidal signal in order to form a first sinusoidal signal applied to the measurement element via at least the first coupling means.
The corrector includes an analog-digital conversion means for converting the analog error signal into a digital signal, the means for pulse-width modulation of the error signal includes a corrector of the proportional and integral type for the digital signal in order to form the intermediate signal.
The linearization means of the corrector includes a means for forming the square root of the error signal or of a signal proportional to the error signal in order to form an intermediate signal and a means for modulating the intermediate signal with a sinusoidal signal in order to form the alternating signal.
The means for forming the error signal is digital.
The measurement resistor for example consists of manganite.